GOT used to be my favorite series of all time, I used to wait for every episode with more excitement than before.
Witnessing such an ending for such a series that everyone worked hard on to make it what it is today honestly broke my heart. As Kit Harington once described the ending it was disappointing.
Without spoiling much about the ending (except for the fact that it is utterly underwhelming) the main problem was in my opinion that the character development throughout the years that made this series quite enjoyable and made us viewers go into the ending with exceptionally high hopes was not used at all. Quite the opposite in fact, the backstory of some of the main characters that made you think they were destined for greatness might as well have been non-existent. The writers seemed to have put together this ending as a last-minute project (while in fact they had plenty of time) and they have remarkably succeeded in creating what can only be described as "the worst ending in the history of bad endings".